CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on Japanese Patent Application No. 09 (1997)-228448, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to a malfunction detection device and method of operation for a wheel speed sensor.
2. Description of the Related Art
A wheel speed sensor for a vehicle is conventionally included in an anti-skid control device, a traction control device for a vehicle and so on. Since, in this case of malfunction of the wheel speed sensor, the control device for a vehicle can not control the vehicle, the malfunction of the wheel speed sensor must be precisely detected.
With reference to the detection of the malfunction of the wheel speed sensor, conventionally a controller compares the direct current flowing in a wheel speed sensor with a predetermined value in order to detect a cut wire or a short circuit of a wire of the wheel speed sensor.
In accordance with the conventional malfunction detection device for a wheel speed sensor, when the controller detects the malfunction of the wheel speed sensor, a real rotation of a wheel of the vehicle imposes a wheel speed signal current on the direct current so that the amount of the electric current momentarily rises to a predetermined value. Therefore, the controller incorrectly detects the malfunction of the wheel speed sensor.
A CR-filter which is formed with a capacitor and a resistance is mainly used in order to prevent the amount of the electric current from exceeding a predetermined value. A conventional malfunction detection device for a wheel speed sensor comprising the above CR-filter (shown in FIG. 5 of the present application) is disclosed in Japanese Patent Laid-Open No. 5 (1993)-133962.
In FIG. 5, a wheel speed signal is transmitted to a control circuit 30 from a wheel speed sensor 1 so as to be input to a micro computer through a CR-filter formed with a resistance R1 and a capacitor C1. The control circuit 30 compares the amount of the wheel speed signal with a predetermined value in order to detect the malfunction of the wheel speed sensor 1.
In accordance with FIG. 5, electronic elements of the malfunction detection circuit 31 except the capacitor C1 can be integrated. However, since the capacitor C1 must have a large capacitance, the capacitor C1 is difficult to be integrated so that the capacitor C1 is separate from the integrated circuit as a discrete part. Therefore, the space in which the electronic parts of the malfunction detection device are disposed for a wheel speed sensor must be enlarged. Further, the malfunction detection device for such a wheel speed sensor will be high in cost.
In order to solve the above problem, the wheel speed signal may be directly input into an A/D converter of the micro computer so that the wheel speed signal passes through a digital filter in the micro computer. Since the wheel speed sensor outputs a wheel speed signal of about 0.about.10 k Hz frequency, the A/D converter must be able to treat a high frequency signal of at least 20 k Hz.
Therefore, the micro computer will be high in cost in order to treat the high frequency signal.